After the ABA–NBA merger, Lucas was selected by the Portland Trail Blazers in the subsequent ABA Dispersal Draft in which the Kentucky Colonels and Spirits of St. Louis players were selected by NBA teams. Portland had traded Geoff Petrie and Steve Hawes to the Atlanta Hawks for the second overall pick, which they used to select Lucas. In the 1976–77 NBA season, Lucas led the Trail Blazers in scoring, minutes played, field goals, free throws, and offensive rebounds. Not only did the team qualify for their first trip to the playoffs that season, but Lucas and teammate Bill Walton led the Trail Blazers past the favored Los Angeles Lakers, sweeping them 4–0 in the Western Conference Finals, and a surprising come-from-behind 4–2 upset victory over the Philadelphia 76ers in the 1977 NBA Finals.

In that NBA Finals series, Lucas asserted his "enforcer" role in Game 2. With the 76ers comfortably ahead late in the game, the Blazers streaked down the floor on a fast break. Lionel Hollins missed the shot, both Bob Gross and Darryl Dawkins went up and wrestled for the rebound, and both came crashing to the floor. As Dawkins ran up court, he threw a punch that largely missed Gross, nailing his own teammate instead. As Dawkins reached mid-court, Lucas greeted him with an elbow to the head, after which they briefly squared off.[6] Both benches emptied and Dawkins and Lucas were ejected. Although the 76ers would go on to win the game and go up 2–0 in the series, Lucas' actions appeared to alter the momentum of the series in favor of the Blazers. Inspired, Portland won the next two games at home in blowouts, then won at Philadelphia, and closed out the 76ers at home to win the series. Lucas remained with Portland until 1980 when he was traded to the New Jersey Nets.

Lucas next moved through several different teams in several years, playing for the New York Knicks, the Phoenix Suns, the Los Angeles Lakers, helping the Lakers to the Western Conference championship series in the 1986 NBA Playoffs in his only year with that team. On Wednesday, December 4, 1985, Lucas made a 60-foot shot at the regulation buzzer to send the game into overtime. The Lakers would go on to defeat the Utah Jazz 131-127.[7] Next, Lucas moved to the Seattle SuperSonics for one year, before returning to the Trail Blazers for his final NBA season in 1988.

In his fourteen-year professional basketball career – two in the ABA and 12 in the NBA – Lucas scored 14,857 points and gathered 9,306 rebounds. He was a five-time All-Star – one in the ABA and four in the NBA. He was named to the 1978 All-NBA-Defense First team, the 1978 All-NBA Second team and the 1979 All-NBA-Defense Second team.

The Portland Trail Blazers retired his jersey number, 20, in a ceremony on November 4, 1988.[8] Lucas was hired by the team as an assistant coach under Mike Schuler and Rick Adelman during the 1988–89 season. In 2005, Lucas rejoined the Trail Blazers as an assistant coach under Nate McMillan.[9]

Lucas underwent surgery for bladder cancer in April 2009.[13] With his health continuing to be a concern, Lucas resigned his coaching position following the 2009–2010 season.[14]

Lucas died at his home in Portland, Oregon, on October 31, 2010.[15] Services were also held in his childhood home town, Pittsburgh.[16] The Blazers honored him by wearing No. 20 patches on their jerseys for the 2010–2011 season.